This invention relates to a current source arrangement comprising N current sources (N.gtoreq.1) for generating N substantially equal currents, said sources comprising N.times.M substantially equal current source transistors. The invention also relates to a digital-to-analog converter comprising such a current source arrangement.
Such a current source arrangement provides the possibility of realizing a digital-to-analog converter by applying the currents of the current source transistors to a summing point via switches controlled by the digital input code or by draining them to a power supply terminal. The current occurring at the summing point then constitutes the output signal corresponding to the digital input code. In such a converter one current source is connected in circuit upon each increase of the digital input code. The converter is therefore monotonic throughout the range of the input code.
A converter of this type requires such a large number of current sources and switches for achieving a high resolution that such a converter can hardly be realized. Approximately 1024 current sources are required for, for example, a 10-bit digital-to-analog converter.
To achieve a high resolution with a limited number of current sources, the least significant bits are often realized by splitting up the current of one of the current sources by means of, for example, a binary current divider. For example, a 10-bit converter can be built up from 8 equal current sources, by using the currents of 7 current sources to form the 3 most significant bits and by applying the current of the remaining current source to a binary current divider to form the 7 least significant bits.
To prevent monotonic errors in such a converter, the current of each current source of the 3-bit part of the converter should be at least equal to the sum current of the 7-bit part of the converter. However, the mutual deviation between two current sources increases as the distance between these sources on the integrated circuit increases. Consequently, the deviation between the current source whose current is applied to a binary current divider and a current source located at a further distance can become so large that a monotonic error occurs. To prevent such errors it is known that, instead of applying a fixed current to the binary current divider, the current of the current source from the 3-bit part located next to the current source which is the last to be connected in circuit is applied to this divider. A digital-to-analog converter based on this principle is known from Digest of Technical Papers 1985 IEEE ISSCC, pp. 32-34. A drawback of such a converter is that a complex switching network of three-way switches is required for switching the currents of the current sources from the 3-bit part.